text:
Are Jeppesen Approach, SID charts in Magnetic North or True North?
Usually, when making charts, they use true north up for their charts.
However, I found that there is a north arrow in jeppesen approach and sid chart.
And.. what north arrow means?
Is it indicating true north or magnetic north?

I would be amazed if Approach charts used anything but magnetic, as thats what you're reading on your instrumwents when using them. But trying to find some actual evidence of that

Idon’t know about Jepp but  just overlayed an FAA chart over the map in ForeFlight and they are definitely oriented to True North.

@JScarry the orientation may well be but the question is about the north indicator/arrow on the chart (if I understood correctly)

All charts are drawn in reference to true north, and arrows or isogonic lines (depending on the type of chart) show how to convert that to magnetic.
The reason is that true north doesn’t change over time, but magnetic north does.
If charts were drawn to magnetic north, they’d have to reframe every chart every few years as the variation changed. However, by drawing them to true north, they just need to update the arrow or isogonic lines, and everything else stays the same.

Is this documented somwhere in the JeppView software?

Jeppesen Do provide some Guidance on the use of their charts but do not explicitly state whether the "North arrow" in use is showing magnetic or true north.
However, I think we can surmise that what they depict is magnetic for two reasons

Instrumentation in use when using these charts (Compass, DI, HSI) will all be reading magnetic so it makes most sense that bearings shown will be magnetic
If they were depicting true, then they would also have included magnetic variance so the user can calculate magnetic.  They do have this on en-route charts, but not on approach plates.

A little late to the party:

All charts are referenced to True North. Therefore the picture we see is True.
On a VFR chart we measure the True Track referenced to ground landmarks, but then we convert it to Magnetic, since we are flying with reference to our magnetic compass or whatever.
An IFR chart is still referenced to TN pictorial-wise, but all the bearings etc are Magnetic, because we are not flying using external references, and the tracks are all pre-defined and ready for us in Magnetic.
The North arrow on the charts (ie Jepp) always points to the TN, no matter which chart you are looking at. Difference is that in En-Route / SID / Arrival charts there is no need to show the Variation (see 3 above). The only calculation needed is in case we have the wind in True (ie pre-flight wx info - Metar-Taf). But remember, the tower or the ATIS will pass to us the MAGNETIC wind and in that case, no calculation is necessary.
The TN and MN (together with the appropriate MV) is shown only on the Terminal (airport) chart for obvious reasons (including reference for the wind calculations etc)

Thx! Your answer really helped me.
I couldn't find any explicit answer on it.


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Glideslope reliable height and PAPI usage
First of all, which height can we rely on glideslope indication?
From what I know, we can trust and use glideslope to Decision Height, not even below Decision Height.
If we use glideslope even passing Decision height, Glideslope signal can become unreliable and it can impose some problem related to signal interference.
I can't find any reference on it. It's just my inference.
Second, PAPI is for a visual glide path.
What I mean is, PAPI provides a descent angle when we see at our eye height at the cockpit.
However, Glideslope provides a descent angle that the airplane's glideslope antenna passes sited angle.
But, according to ICAO Annex 14 vol 1, aerodrome design, it says that PAPI collocated with ILS should be installed that it coincidences with the glideslope indication.
So, I think below decision altitude, relying on PAPI indication will make the airplane descent even as same as glideslope descent angle.
I know that every airplane's eye height is different. And, in some airplane that has a lower cockpit than the standard operating airplane at the airport, if the pilot uses papi after descent altitude, the airplane will need to climb for papi glidepath which should be higher than glideslope.
However, is using papi after decision altitude not okay even for standard operating airplanes?
What I want to say, in every case, only following glideslope's descent angle even after the pilot can see PAPI is allowed or not.

I couldn't find anything in the FAA regs that explicitly allows or disallows usage of either the papi or the glideslope below decision height. The regulation that seems interesting to me related to this issue states that you must always be able to make a descent to landing on the intended runway using normal maneuvers and a normal descent rate. Since the glideslope will take you down to the 1000' bars on most instrument runways, I see no issue with following the glideslope signal once visual. However, especially in smaller airplanes, it may make more sense to deviate above the glideslope to deploy flaps or other drag devices and adjust your descent profile once visual with the landing area.

Isn't it that same GS signal that guides the aircraft to the RWY surface in a CATIII ILS approach?

I read the question as related to glideslope (ILS) vs PAPI indication lights.

The OP said the GS was unreliable below DH. My question was whether the GS provided vertical guidance to the RWY surface for CATIII ILS approaches.

Although Loc is used for lateral guidance all through the landing, at a certain point rad alt takes over for controlling the roundout and flare, which is why valid rad alt is necessary for Cat II and III approaches.

@RetiredATC If the ground equipment and avionics are certified to Cat III, yes, but not true for ILS generally.

Coincident does not mean the same angle, it means that they cross. The decision height has little or nothing to do with the guidance signal, the DA is designed to allow for obstacle clearance even with poor altitude-instrument accuracy and some pilot/plane limitations(like response time) during a zero visibility missed approach. Often DA is not determined by the approach side but is set so that terrain and obstacles can be avoided during the missed approach procedure with a minimum assumed angle of climb and the legal maximum altitude instrument error. Both vertical and horizontal obstacle clearance is considered. Instrument systems require much greater minimum horizontal clearance than visual systems as there is more chance for aircraft deviation and less chance for emergency corrective manuevers.
PAPI is an assistive guide, not a required path. Pilots of large aircraft such as the 747 know that they have a high line of sight and they will use three white one red as a PAPI target instead of 2 white two red.

The fix for a high cockpit like the 747 is a 3-bar PAPI, which gives you the same angle (typically 3 degrees) to a point slightly farther down the runway. Flying 3-white/1-red will give a steeper angle (not what the 747 needs) to the same point on the runway (which is what you're hoping to avoid).

Yet that is exactly what they do on a normal basis. It can actually lead to a shallower angle because the point where they switch from electronic glide slope to visual slope indicator will have them at 2r-2w and they reduce the mean decent angle to achieve 1r-3w before landing. The difference in 2-2 vs 1-3 is only 0.5 degrees anyway. (From memory, not for use in critical engineering.) Also the design of new final approach slopes must consider height of the aircraft in obstacle clearances during the visual segment, in such case a steeper angle may be required for the taller aircraft.

The VGSI (PAPI or VASI) should be coincident with the ILS GS, and ideally both will have a Threshold Crossing Height (TCH) of 50ft. Exceptions will be noted on the approach plate.
There may be a slight additional discrepancy due to the difference in position of the pilot’s eyes vs the the GS antenna. This is something pilots of such aircraft learn to compensate for during training.
You should not use the GS below the charted DA/DH because there is no guarantee it has been validated below that.
In contrast, you can use the VGSI from as soon as you can see it (which may be several miles away on a clear day) all the way down to the flare, which will be guided by the radar altimeter in larger aircraft.

You cannot use the VASI or PAPI as soon as you see it.  It has been validated for obstacle clearance out to only 4NM (VASI) or 3.4NM (PAPI) with a ±10° from centerline.  It is a common misconception to think if a pilot can see the VGSI lights, they are clear of obstacles.  See FAA AIM 2-1-2.

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FAA Handbooks Copyrights
I want to write some articles about aeronautical knowledge on my blog.
However, I heard that FAA's all publications including handbooks (e.g IFH, AFH, PHAK, etc.) are copyright-free.
But I can't find any reference for it.
Are there any legal issues if I use these ones for commercial use?
Plus, all photos and pictures included in handbooks are also okay for this purpose?

The publications you listed are in the public domain in the U.S. because, as works of the Federal Government, they are not eligible for copyright protection. You may use them commercially, subject to the same restrictions (such as civil liability) as use of any other kind of material.

Makes sense, since we’ve paid for them…

Pretty sure the same is true for most gov't agencies, e.g. NASA, NOAA, CDC, etc.  Maybe not the ones that deal in classified national defense stuff, but most of the others.

@DarrelHoffman It's true for almost all works produced by the Federal Government. Note that lack of copyright does not equate to must be published or disclosed. Classified material is a different thing. It's just that works of the Government are by law not eligible for copyright, with a few niche exceptions.

Perhaps worth mentioning is that those works are only in the public domain in the US. As per the linked article: "This act only applies to U.S. domestic copyright as that is the extent of U.S. federal law. The U.S. government asserts that it can still hold the copyright to those works in other countries." - if OP is not from US, this status is not as clear.

@jaskij I can't find any evidence (and have never heard) that the FAA asserts any foreign copyright claims on its publications. The government only reserves the right to do this. If there's evidence to the contrary, I'm all ears!

This is not the right reasoning. While works of federal employees are public domain, it is not necessarily true that everything published by the FAA was written by FAA employees. If the FAA hired someone to draft their publications, then bought the rights, that (generally) is copyrighted, the FAA just has rights/a license to it.

@fectin That's true, but all of these works were produced by the FAA itself and are not copyrighted. (I've even met one of the authors of the AFH and have a signed copy.)

@TypeIA you are correct in this, I also am not aware of any US agency asserting that copyright, but the right is there. In which case, if the OP is not from the US, using those materials is itself IMO not fully clear legally. IANAL though. Here's a relevant question from the law site, although it doesn't fully answer this.

The FAA has a FAQ that mentions this:

All digital products published by the FAA are in the public domain and are not copyright protected. Therefore, a written release or credit is not required to incorporate them into your own digital products.

So copyright is no problem.
Another possible issue is liability. You may be opening yourself up to liability if you publish out-of-date or incorrect aeronautical information. For example, if you publish a chart on your blog, you should take care to clearly mark it "Not for navigation" unless you want to keep it up-to-date and accurate.
It should be obvious, but you also shouldn't claim that you made the materials or that the FAA in any way endorses your blog.

I think you're right, but that particular FAQ is about digital flight data (charts, nav info etc.). I tried to find something specifically covering the Handbooks, but nothing immediately turned up.

@TypeIA From a practical standpoint, you may also be reassured by the printers who print out the PHAK/AFH/etc. and sell it for money without the FAA getting mad at them.

I don't need reassurance, for me the fact isn't in question but the cited FAQ isn't applicable to the question (for other readers).

@TypeIA As far as I can tell, this is the clearest statement that the FAA intends that their digital products are intended to be public domain. The FAQ is for their charts, but it does say all digital products produced by the FAA.

You shouldn't claim that you made it, but it's probably not illegal. In the same way that I can claim that I wrote Pride and Prejudice.

@fectin: Indeed, plagiarism isn't directly a crime in and of itself.  Some countries don't allow moral rights (e.g. to attribution) to be waived, so nobody could stop the original author from suing someone that falsely claimed authorship, but that would still be something they have to actively choose to do.  The US isn't one of those countries (so the FAA may have gotten the authors to waive those rights).  And FAA publications rarely(?) have just one human author anyway.

@PeterCordes Plagiarism isn't a crime in the United States. I wouldn't assume that's the case everywhere.

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Questions with A220 control logics
I'm gonna take simulator check for new-hired co-pilots.
The company will do simulator test in A220 for new-hired copilots. Their applicants have no experience in jets, have experiences only in single or multi engine props. (like C172, PA44)
Thus, there's no way to experience A220, cuz that model is not famous, so there's no simulator programs or place to experience.
But I found out that much differences are existing in A220. I'm trying to practice with MSFS or XPlane with A320 / B737 and A320 FTD.
There are several curiosities in control logic.

A220 has TOGA switch in the thrust levers, underneath the handle of it. But, in many videos taken in the cockpit of A220 while taking off, I found that pilots do not push toga button. But, I can see thrust levers are automatically set to Takeoff Thrust position. How can they do that? Is there any button that pilots should press while taking off?

A220 has weird control logic, speed trim. I failed to find any jets who have speed trim switch. Thus, I understood speed trims will fix the trimmed speed, so pilots have to change their speed trims only they want to change their target speed. But, you know B737, B747 or other boeing jets who have trim pilots should continuously change their trim while changing their attitude. In this part. when changing the attitude, what logic will be executed in A220? In speed trim at 210 KIAS with 0 FPM and 50%N1, in this situation, if a pilot commands back pressure on sidestick without changing thrust, airplane will decelerate. In turn, if a pilot returns his sidestick to neutral position, the airplane pitches down by speed trim system? Thus, if a pilot wants to climb, he should command back pressure on sidestick plus increase thrust?

Question 2 is the hardest part to understand ;(
Thanks for answering.

With an autothrottle system, you arm the autothrottle system with a button on the glareshield, and just advance the thrust levers until the AT system takes them over. There are servos in the console that drive the trust levers. The AT system sets power for you and sets the Flight Director to Takeoff Mode. You would only use the TOGA buttons if taking off without autothrottle.
The trim speed bug (the blue bug on the speed tape), was a new feature for the C-Series program, intended to reduce workload when hand flying. You trim to a speed for a given configuration (speed is a proxy for angle of attack) in any airplane. Jets with trimmable stabilizers don't trim like a light aircraft with an elevator tab. Instead of the control column moving to a new hands-free position when you trim as with a tab, the neutral position of the column is always in the same spot. So if you are holding elevator, you apply trim, and back the elevator off to neutral and see if the trim was enough. If not, you reapply elevator and trim again, repeat. In actual practice if you are holding the column aft of neutral, you blip the trim and ease it forward at bit, blip again and ease it forward some more, and repeat until the pitch attitude is held with the column at neutral.
Same with a side stick. Neutral is always at the same spot.
With the trim speed bug feature, you run the blue bug with the trim switch to the speed you want on the speed tape, and the FBW will set the stab to give hands-off flight at that speed for you, taking out the guesswork and gradual tweaking to get the trim right. If you're hand flying and slowing to 200kt, you don't have to pitch to 200kt and then play with the trim switch going blip-blip-blip until you can let go of the stick and it'll hold the speed/attitude, you just drive the blue bug to 200kt and the airplane sets the stab for you for hands off flight at that speed.
The airline isn't going to expect you to know anything about the A-220. You will be told to perform a series of maneuvers, knowing that you don't know the airplane, and the evaluation pilot sitting with you will do stuff for you that are aren't expected to know, and will likely tell you, "Don't worry about this function or that function, I'll take care it; you just fly". They will be looking for basic flying skill, and also observing how fast you catch on to something new and unfamiliar, plus any signs of attitude or personality issues.
That being said, if they see that you've gone to some trouble to learn the aircraft, it won't hurt, so learn as much as you can all the same.

Wow! Thanks for detailed answer. But, you know I will let not to use autothrottle during the whole checkride. Thus, my curiosity is when my airplane is trimmed at 200 KIAS, and I've commanded to increase airspeed to 210 KIAS, if I don't increase thrust and only applies nose down trim (to increase trimmed airspeed), what will happen? Dose airplane maintain previous pitch attitude or command to pitch down for acceleration? (Presumes that Autothrottle is OFF)

And.. Second one, when I want to climb with 200 KIAS (trimmed with 200 KIAS), what if I don't increase thrust and only apply back pressure on sidestick for changing pitch attitude and return to neutral position, does the airplane automatically pitch down to maintain 200 KIAS or maintain newly increased pitch attitude?

If the thrust is constant and you raise trim speed from 200 to 210 kt, the airplane will pitch down to maintain 200 kt. It works the same as a regular stab trim; blip nose down trim, and the plane will pitch over to maintain a new speed based on the changed stab setting.  It's just that with the C-Series trim bug, you know exactly what revised trim speed will result. In the C-Series there is a compromise between the Boeing pilot-more-in-the-loop FBW logic anthe Airbus pilot-less-in-the-loop FBW logic. I don't have a A-220 type rating, but spent a bit of time in the development sim years back.

If trim speed is set to 200kt, and you pitch up without changing thrust, and the plane slows down, if you just let go, it will return to 200kt. It doesn't auto trim to a new speed if you are hand flying it.  You might get briefed on the ins and outs of the trim.


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When is it appropriate to foam a landing strip? (example JB292)
Background:
I recently watched the Channel5 documentary on JetBlue 292 with their famous sideways landing gear. At around the 8:00-mark you see footage of the actual landing: Main landing gear intact but nose gear twisted. It was mentioned in the documentary, that complete nose gear failure and subsequent friction and possible fire were a main concern.
This leads me to my question: When is it appropriate to foam a landing strip?
Prior to actually thinking about it, I just assumed, that whenever friction/fire was expected (damaged/retracted landing gear), one might want to foam up a runway, to provide extra cooling and fire suppression for the stricken aircraft. So, in my non-aviator-thinking, this would have been a textbook example of wheren to foam a runway. But in the documentary (7:48-8:10) it is clearly visible that there was no foam - from the point of touchdown all the way until the complete stop.
To rephrase my question in two parts:

What is the reason for JB292's landing to be without foam?
If the equation "Bad Landing Gear = Foam Needed" does not hold, what are the policies of when to apply foam?

It is appropriate to foam the landing strip if the landing strip is on fire.

This is not an answer because I don't have any sources to cite at the moment, maybe someone else wants to turn it into one. If my memory serves, usage of foam has significantly decreased after an incident where a passenger was hidden by the foam, and subsequently run over and injured / killed by a fire truck.

@Mark’s comment seems silly but actually very true on aircraft carriers.

Covering an entire runway in foam takes a lot of foam. There may not be any left for post-crash fire.
Foam reduces surface friction and brake action, friction isn't always bad, it makes it so that things stop. If you apply foam, you may overrun the runway because you are sliding.
Directional control is difficult on foam.
Evacuating people into foam means they may slip/fall.
Fires are not difficult to deal with if the aircraft is intact. Historically evacuations are very effective (FAA requires the entire aircraft can be evacuated in 120 seconds with 50% of the exits available).
AFFF isn't that great for the environment
It is really difficult to wash off in that volume, leading to delays in moving the aircraft and reopening the runway

Technically this is called a Foam Path and was the FAA recommendation back in the 60's, but the recommendation was retracted in '87. In 2002 the FAA released Cert 02-04 recommending against the use of foam paths specifically for items listed above.
Belly landings and failed nose-gear landings rarely result in post-crash fire provided the pilot can keep the aircraft on the runway.

Here is an example of one airport in Warsaw that did foam the runway (incident is LOT Flight 16):

Notice how a couple passengers did slip and fall in the evacuation.

Thank you for this amazing answer! Since you linked the UnitexExpress and LOT examples: Do you know, why one had fome while the other in a similar situation had not? Does this maybe just depend on the opinion/mood of the airport people handling the emergency?


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Is this a 747 or a 380?
I got a video of a 4-engine jet landing at LAX on December 28, 2018 at 12:40 PM.  Here's some screen shots from the video:

I think it is a 380, but....

A380. The A380 has 2x3 trucks on the body gear (as seen in your pic), the 747 has 2x2 trucks.

That's the A380 with its 6 wheel inner main landing gear. 747s don't have 6 wheel boogies. The winglet is another good hint.

Also the A380 has the flight deck on the lower deck while on the 747 it is on the upper deck. In this case you can see the nose and the flight deck are fairly low on the hull of the plane.

That is an Airbus A380.
There are probably many distinctive features. One of the easiest to see (in your third picture) are the wingtip fences:

(British Airways A380 wingtip, Wikimedia Commons)
The 747 has no wingtip device (-100 to -300 models), winglets (-400 model) or raked wingtips (-8 model).
You can also clearly see the strong dihedral on the inner wing (see e.g. this answer for an explanation). Other distinctive features are the ram air outlets (easily visible in the second picture, see this answer for more details) and of course the main body landing gear with 6 wheels as mentioned in the comments:


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At what speed will the crankshaft be rotating if each cylinder of a four-stroke cycle engine is to be fired 200 times a minute?
Well the answer is 400rpm ; but I don't understand how.
Here's my thought process:
4 stroke engine would mean every piston is in a different stage (intake, compression, power, exhaust)
Which means 200 power strokes every minute which would suggest 200rpm.
Could you tell me where I went wrong? Thanks in advance.

It doesn't mean every piston is in a different stage. It does mean that the piston is firing once every two rotations though. Think of how a four stroke works and how many full rotations it takes to get to the power stroke.

In your third paragraph, I think you're getting "4 stroke" confused with "4 cylinder". In a 4 stroke 4 cylinder engine, then, yes, each cylinder would be in a different stroke. (Note that the same would be true of a 2 stroke 2 cylinder engine.) But if the number of strokes doesn't equal the number of cylinders, that's not going to be true.

Careful it is not a trick question. Some four stoke engines spark (or fire) at the top of the exhaust stroke, especially on motorcycles and marine engines.  This is know as wasted spark and may or may not be a red herring if this is an interview question.

@mckenzm the technology was popular in car engines in 1990-2010 as well. There is a spark, but no firing. Well, unless the timing is WAY late in which case you fire in the intake tract. The bastards introduced the plastic intake assemblies roughly the same period...

This is scarily similar to the old jokes about  doubling the oven temperature to halve the baking time

I'm not sure how this is an aviation question.  This is strictly a question about internal combustion engines.

In fact, this question is lifted directly from some mechanized agriculture flashcards on Chegg...

In a four-stroke engine, each cylinder goes through the following stages during one cycle:

(image modified from FAA Pilot’s Handbook of Aeronautical Knowledge)
Let's look at the motion of the piston and the crankshaft during each stage:

Intake: the piston is moving down and the crankshaft performs half a rotation
Compression: the piston is moving up and the crankshaft performs half a rotation
Power: the piston is moving down and the crankshaft performs half a rotation
Exhaust: the piston is moving up and the crankshaft performs half a rotation

So during one cycle, the piston has moved down and up twice and the crankshaft has performed 2 full rotations. Therefore, when one cylinder is fired 200 times, the crankshaft will have rotated 400 times.

Ah, the good ol' "suck, squeeze, bang, blow"...

Note that in half the strokes, the engine is doing work on the crankshaft, but in half the crankshaft is doing work on the engine (the power stroke is so much more than the other ones that the net work done on the crankshaft by the engine is positive). So 200 firings per minute means 200 rotations' worth of the engine doing work on the crankshaft, and 200 rotations' worth of the crankshaft doing work on the engine.

@Acccumulation 3/4 work done by crank, 1/4 work done on the crank. Intake, compression and exhaust all require force input from the crank, only power stroke drives the crank.

@PcMan ... unless you have a wide open throttle and a supercharger. Then the intake does work as well.

And as a bonus question: how many revolutions would it be for a Wankel rotary eingine?

It's a little like music (if the engine is running well).  Each stroke (up or down) is a half beat.  So, 4 strokes is 2 beats.  The power stroke is a half note, followed by three rests.  In order to get 200 notes per minute, you need to play 2x the number of beats per minute.
Although the number of cylinders is not specified, if each cylinder fires at a separate time, the answer still holds, but now you have a symphony.  Each cylinder must make 4 ups and downs (2 beats) to make 1 note.
Each beat requires a complete revolution of the crankshaft for that particular cylinder.  With more than 1 cylinder, at the same rpm, you'll be hearing more notes per minute, which allows a larger prop, or more blades.
With a jet, it's a constant hum, but the thrust is still dependent on fuel consumption and efficiency per unit time for a given airspeed.
For 200 "fires" per minute it should look like this:
| ● ○ | | ○ ○ |
Each "measure" is turn of the crankshaft.
But for a 2 stroke engine, you would be correct:
| ● ○ | | ● ○ |

That's quite the poetic way of putting it. I like it!

On the 4 stroke cycle the cylinder fires every two turns of the crank.
For that cylinder to fire 200 times, it requires 400 turns of the crank.
That's for a single cylinder engine.  If there are 4 cylinders evenly spaced, one of the 4 cylinders fires every half turn.
So for every full turn of a 4 stroke 4 cyl engine, two cylinders have fired; that is, two power pulses have occurred.
Therefore, if the crank turns 400 times in a minute, each cylinder will have fired 200 times, but if there are 4 of them, there will have been 4 x 200 = 800 power pulses in total in that minute period.
If you have a two blade prop, two blades pass by with each revolution, and there are 2 cylinder firings with each revolution. So when you spin the prop by hand, you come up on the compression stroke of one of the cylinders with each half turn of the prop, or each blade at the same clock position (props are usually installed to place the blade such that the top of a compression stroke is reached at roughly 10 o'clock blade position when standing in front facing it).
The result is the sound frequency on a 4 cyl two blade engine is same for both the propeller blade beat and engine exhaust pulse frequency (roughly 60-90 Hz from cruise to full power, and about 13 Hz at idle), which is the dominant frequency in the cockpit, and why ANR headsets, which work best at very low frequencies, are so much more effective than earplugs in airplanes.

Good info on ANR headsets.

A few years back I put an ANR retrofit kit the Dave Clarks I bought new 45 years ago (they have the old dynamic mic), and it works pretty good, but I'd like to give a set of Lightspeeds a try.

Not sure if the number of cylinders should even be considered for a question concerning an arbitrary cylinder. The Corsair had what, 27 cylinders?

This is just relating the concept the OP asked about into a configuration that 99% of new pilots encounter first.

Your misconception is about each piston being in a different stage.  That's only true for balanced 4-cylinder engines.
The "Stroke" refers to the piston moving full travel up or down. You get 2 strokes per engine revolution, so 4 strokes is 2 revolutions.
It's all about scavenging, you see. Removing the burnt exhaust gases and re-filling the cylinder with fresh air and fuel.  On a 2-stroke engine that happens practically as a footnote at the very bottom of the stroke.
But on a 4-stroke engine, they dedicate 2 whole strokes to scavenging.  After the downward power stroke, the upward exhaust stroke (with exhaust valves opened) forcibly expels spent exhaust gases. Then comes the downward intake stroke (with intake valve open) drawing in fresh air and fuel. The next upward stroke is the compression stroke as in a 2-stroke... then the next downward stroke is the power stroke and the process repeats.
There are many advantages to using 2 whole strokes for scavenging.

The cylinder itself is used to pump the air and exhaust, so no external "pump" is needed to force intake air in, and exhaust out. On simple 2-stroke engines, they use the backside of the cylinder as the air pump, necessitating the use of "2-cycle oil" mixed into the fuel.   On large engines, they need a Roots style blower, or a turbocharger with a gear drive and overrunning clutch to forcibly drive the turbo compressor when exhaust energy is not enough.
On a 2-stroke, the piston must be tall, so it can block the intake port in all positions except the bottom.  That limits piston stroke and engine size.
Near perfect control of gases.  2-stroke scavenging is plagued by poor performance - the engine either forces in too little air, leaving some exhaust in the cylinder; or forces in too much, pushing air-fuel straight into the exhaust pipe, where it can cause trouble.  (and on an airplane, throw off your range calculations, causing unexpected stops at best!)  This imperfect gas control would vary by engine RPM, throttle setting and environmental conditions (temperature, altitude, humidity).

When the engine completes each stroke, the crankshaft rotates 180 °, that is, when the engine completes a working cycle, the crankshaft rotates twice, and the flywheel and crankshaft are a whole, so the flywheel rotates twice and the camshaft rotates only once, because the number of teeth of the camshaft is twice that of the crankshaft. In a working cycle, the engine works only in the working stroke. In the other three strokes, the crankshaft is driven by the flywheel's own rotational inertia to overcome the mechanical running resistance.

4 stroke engine would mean every piston is in a different stage
(intake, compression, power, exhaust)

That's only true if the total number of cylinders is four or less.  If you are implying that all of these different strokes are represented at any given time, then there would have to be four cylinders, but technically, you could also say of a two-cylinder or three-cylinder engine, that with a four-stroke cycle, "every cylinder is in a different stage" at any given time.

Which means 200 power strokes every minute which would suggest 200rpm.

No, given your apparent assumption of four cylinders, 200 power strokes every minute would suggest 100 rpm.  Because each cylinder fires once every two revolutions of the crankshaft, and there are four cylinders, so every half turn of the crankshaft, a cylinder fires.

Could you tell me where I went wrong?

See above.  But more fundamentally, you misread the actual question ("At what speed will the crankshaft be rotating if each cylinder of a four-stroke cycle engine is to be fired 200 times a minute?") to mean that the total number of firings of all the cylinders together was 200 per minute, when the question was actually stating that any one given cylinder fires 200 times per minute.  Which is why we don't need to know the total number of cylinders.


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Would a Tip Jet be more efficient or practical the bigger/longer it gets?
In summary, a tip jet is a jet nozzle at the tip of some helicopter rotor blades, used to spin the rotor without much torque.

Source of the picture (it is an article)
One of the types of Tip Jet is using a compressor to inject air and fuel in a duct (as shown in the image), but even without the compressed air, if you elongate the rotor enough, in the right speed, the air could be compressed and mixed with the fuel (and thus, create thrust) without the need of an external compressor.
But what made me make this question was this video explaining about the SpinLaunch, where it is explained that the tips get even more heavier the faster it spins. Also as shown in this video, when you rotate a hollow "duct" fast enough, it starts to suck air.
In summary, what I could understand of the centrifugal force in a rotor is that the more it accelerates, the "heavier" the tips get, and thus, the air could be compressed with this phenomena.
So, I thought it could make sense to make something like this, but I don't know if the speeds with the air friction would allow the rotor to spin fast enough to be able to compress air and fuel.

The question:
Would a longer tipjet be more efficient than the previously made helicopter-sized Tip Jets?
With all this in mind, and with the fact that tip-jets were already explored enough through the years, it would be possible to answer this question?
Or even if you were able to do that, a smaller helicopter would be more efficient anyway?

Judging by the Fairey Rotodyne, tip jets are pretty efficient at deafening people. "From two miles away it would stop a conversation." said Hawker's chief test pilot.


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Replacing cyclical control with tilt-able rotor using EV motors and servos
I understand the traditional reasons for cyclical control (with swash plates) on helicopters; ie, avoid gyro effect, shaft can be fixed.  However, if the power plant was a modern EV motor, and collective pitch was achieved by servo motors connected to the blades, could a design that tilts the whole rotor have some advantages?
I am thinking of this design for light units, or drone types which would carry less than 1,500 kg of payload.
Would the removal of the swash plate and flapping blades provide tangible benefit; ie, reduced engineering complexity and manufacturing cost.

Do you really mean 1,500 kg payload? That is quite a good sized helicopter. And how do you plan to engineer the electrical power connections to the motors?

Related: why-does-a-helicopter-need-cyclic-control-rather-than-just-tilting-the-whole-mai

In the fully articulated rotor, the effect of the conventional cyclic control via swash plate is to tilt the entire rotor disk, as defined by the tip-path plane.

Tilting the rotor would not work!
The cyclic (and you could replace the swashplate with electric motors, but it would still be cyclic controls, just less reliable) works by shifting the centre of lift over the area covered by the rotor, and can shift it by significant fraction of the blade length. This is necessary to compensate for the effect of forward flight, which shifts the centre of lift significantly towards the advancing blade (which creates pitch up moment).
By tilting the shaft you could not create nearly as large moments as you can with cyclic. The centre of gravity isn't that far from the hinge, so you'd need larger angle than the geometry allows. With cyclic, the large moment is either created during rolling or pitching or for maintaining speed and the fuselage follows, so the tilt isn't large.
You would also get the full consequences of the gyroscopic effect. The helicopter would tilt to one side to accelerate and to the other side to decelerate! That is because the tilt just creates a moment on the rotor, but the rotor is still a gyroscope that still tilts with (a bit less then!) a 90° lag. The swash-plate deals with this simply by mounting the riders corresponding angle ahead of the blades, with the ultimate effect that the rotor is always trying to align itself with the swashplate (minus the effect of speed).
In fact, many helicopters (e.g. the UH-1 Iroquois (Huey) / Bell 204/205) have teetering rotor and in these the fuselage can't apply any moment on the rotor at all, because (as the name suggests) the pair of blades is allowed to freely teeter on the hub, and the fuselage is just freely hanging below it. Here the property that the rotor is aligning itself to the swashplate is crucial for preventing it from striking either the fuselage or the mechanical stops and getting damaged—when the pilot pushes the cyclic, the rotor starts to tilt, but as it aligns itself with the swashplate, the pitching moment will reduce until the fuselage catches up and the swashplate tilts with it some more.
In fact in all helicopters the rotor is free to tilt in response to the aerodynamic forces and the fuselage is mostly just hanging below it. When the blades have offset flapping hinges (fully articulated rotor) or rely on flex (hingeless rotor) some moment is transferred directly to the shaft, but it is always relatively small compared to just pulling the hub to the appropriate side. Beside dealing with the lift asymmetry due to forward speed, this free flapping of the blades is also necessary to deal with turbulence and similar disturbances.
So no, you can't tilt the rotor mechanically by adjusting the angle to the shaft, you have to do it aerodynamically, and the swash plate is the mechanically simplest mean to achieve that. Separate electric motors would just add more points of possible catastrophic failure.

You are pretty much describing a fly-by-wire rotor control system.
You could certainly do that by using a slip-ring electrical connection at the base of the hub through which you power and control electrically powered servos that regulate blade pitch, with an elastomeric hub allowing for lead/lag and flapping as with most modern helicopter rotors.
With computers monitoring blade position, and controlling blade angle to control the disc plane's tilt in response to commands, there's no reason you can't achieve an "electronic swashplate" through sensors, servos, and software.
A much cruder version of such a thing was done with the Curtiss Electric Propeller in the 30s (an electric actuator driving collective blade angle through simple analog electrical commands in that case, powered through slip rings).
The challenge is certifying such a system in a helicopter that carries people. The amount of multi-channel passive redundancy required, in the control, sensing and servo elements, to reduce the risk of disaster to an acceptable level, makes it too expensive for the benefit achieved in a normal helicopter up to this point.
For a drone however, you don't have that as much, other than the risk of it falling on someone's head, so I would not be surprised at all to hear that somebody is developing such a system for an un-manned helicopter today, and systems like that in regular helicopters will certainly be commonplace is 20 or 30 years.

Thanks John.  I think you're describing where servos undertake the role of the cyclical pitch operation from a swashplate.  I am thinking more about only varying pitch for collective lift.  So for forward movement, the whole motor/shaft/hub are tilted in direction of flight.

Even easier.  You only need a single servo motor in the hub to operate the blades in pitch collectively. You're describing the Curtiss Electric Propeller from 1940, but with electronic digital control instead of analog.

You need servo for each blade, or a swash plate. Otherwise as soon as you start moving, you run into trouble with lift differential between advancing and retreating blades...

Tilting the whole rotor assembly, even with a small helicopter, the gyroscopic forces would be notable. In addition to that, the maneuverability would be quite limited, since achieving enough force to rotate and roll the helicopter, the rotor shaft would have to tilt quite a bit.
And: in flight you would still need to compensate for advancing and retreating blades lift effect, I do not know for sure, but I think you would run into all kinds of problems there if you omitted the swash plate arrangement and implemented rotor shaft tilt to handle this specific problem (if I understood your concept correctly).
If you plan on using single big rotor with servo motors at the root of each blade, why not simply (lol) program a control logic that takes on the task of the swash plate assembly and adjusts every blade independently. If reliable enough servo motors are available (I imagine they are), this arrangement would be quite simple and if more than three blades were used, also reliable and redundant. The program driving the servos might be a nightmare to develope though

Thanks for the answer.  Would the gyroscopic forces affect arrangements like the Osprey tilt rotor?  I know the blades are as big as a helicopter, but they seem significant.

Yes @SteveClancy , but Osprey has two counter rotating rotors, so gyroscopic forces pretty much cancel out.


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Is Sapphire Stone Used as Windshields on Military Aircraft?
The optical clarity of of colorless sapphire stone and its unique refraction of light and strength for example, is not comparable to the clarity, etc. of amorphous/glass laminates.

I think military canopies are all acrylic or polycarbonate. They're tough and relatively easy to manufacture.

See this answer?   https://aviation.stackexchange.com/questions/21802/what-kinds-of-materials-are-used-for-fighter-jets-glass-windshields

If a combat plane has a windshield, it'll be of laminated armour glass, similar to the glass used in an armoured trucks.

"Not comparable" meaning much better, or much worse?

Sapphire(which is basically aluminum oxide with impurities that give it a blue color) wouldn't be used for windows, since it doesn't occur in large enough pieces, and the impurities give it color.  (Many other gemstones are also aluminum oxide, with different impurities.  If it doesn't have the right impurities, it's not sapphire.)   Fused aluminia (manufactured) is available, but I think fused quartz is more common.  IIRC, the Space Shuttle's windows were made of it.

Synthetic sapphire is used for the the front of the iPhone,  I'm no aware of any factory manufaturing large enough pieces for use as airplane windows.

@Jasen The iPhone uses Gorilla Glass for the screen. It only uses synthetic sapphire on its back camera lenses.

@Jasen: It's not "synthetic sapphire", whatever lies Apple's marketing division may tell.  It's fused alumina.

"The front windscreen and canopy [of an A-10] are resistant to small arms fire." https://en.wikipedia.org/wiki/Fairchild_Republic_A-10_Thunderbolt_II - "the canopy and windows [of an Apache] are otherwise unrated against ballistic threats." https://en.wikipedia.org/wiki/Boeing_AH-64_Apache - for some reason I always remember being told they were cultured sapphire.... Not getting shot at in a Apache is part of its charm, just like getting shot in an A-10 is.


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Why do planes not employ a non-technological banking indicator?
I apologise if this is a very daft question with an obvious answer.
I've been enjoying Air Crash Investigation episodes and there have been a few where the pilots, through differing training or faulty instruments, assume they're banking in one direction and try to correct it only to make the problem worse.
I understand it's difficult to get a read of orientation when in an airplane, especially with low visibility or darkness for example so my daft question is why don't they employ something like a marble in a tube across the dash which could clearly indicate banking direction without technology/sensors etc?
Even reading the question back it seems absurd, but part of me thinks there might be an interesting answer.

That’s a slip indicator, is it not?

@ymb1 Absolutely fascinating, thank you. I don't have enough maths and physics knowledge to fully understand but it really is interesting. Can I ask what would happen to a marble on a flat level upon banking or would that be a separate question / answered many times already?

Check this out. Bob Hoover answers your question. https://youtu.be/V9pvG_ZSnCc

Have you noticed that when you are in an airliner, and it banks into a turn, that the liquid in your drink stays parallel to the floor, and other than a sensation of maybe rolling a bit, you can't tell you're in a turn if you can't see outside?   In a maneuvering airplane, centrifugal forces from turning, and gravity, create a net force that is always perpendicular to the floor.  So the "down" you feel is always down relative to the airplane's floor, not relative to the real "down", the ground below.
A physical inclinometer like you describe won't work while moving because of those forces, unless the airplane is skidding sideways, which it normally isn't.  Airplanes actually use such an inclinometer, but it's used for detecting that skidding motion, which is bad to have because flying sideways is inefficient.
The only thing that can indicate where the earth actually is, while the airplane is moving around, is a gyro, so instruments that indicate bank and pitch angles, by presenting a virtual horizon for the pilot to use when he/she can't see outside, depend on gyros, either mechanical or electronic (using lasers).
The struggle for the pilot is the conflict that occurs in the inner ear when it senses motions that aren't being confirmed by the visual picture (an outside horizon) except for the instrument indications.  Unless well trained to ignore your head and trust the instruments you see with your eyes totally, you get dizzy and disoriented, and it can get so bad that you lose control, like what happened to Kennedy.


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Why did this FedEx flight (FDX1311) take such an indirect route multiple times?
I was looking at flights from DEN -> MEM, and saw this extra-long route that FDX1311 took last night (9 Aug 2021)

Thinking they were probably just avoiding storms, I scrolled down and saw there were 3 other DEN-MEM flights under that flight number, all which also took the same extra-long route.

What's the purpose of this extra long route? It's more than double the straight-line distance!

Purpose
FDX1311 is a daily scheduled flight, called a flying spare, or more colloquially a sweeper. It's a provision to remedy unplanned events which would otherwise prevent customers shipments to reach FedEx Memphis super-hub before dawn, and eventually to prevent shipments to be delivered in time.

Fedex sweep flight 1311, Youtube
Route and reuse after landing
The flightpath is designed to allow the aircraft to be diverted to several airports in the US south area where FedEx has an important activity. When reaching Memphis airport, the aircraft becomes available for dispatching shipments from the hub.
Use cases
FDX1311 is part of FedEx contingency plan which, according to Fortune, comprises five aircraft to cover the whole US territory (to be compared to the 600+ aircraft fleet).
Diversion is triggered in quite different cases:

To fix unplanned maintenance. Aircraft availability and safety involves preventive maintenance and unavailability is well scheduled. But unplanned failures or damages still occur, some shipments can be left without a carrier. The spare aircraft will fix this problem.

To fix volume prediction errors. The number of containers can be predicted as soon as the shipments are collected, and aircraft can be dispatched accordingly for the next 24 hours. However there can be mistakes. Mistakes involving large volumes/weights cannot always be fixed with the planned aircraft. Containers in excess may be left at the warehouse. They will be collected by the spare flight.

To collect forgotten shipments. Errors may occur during containers preparation or loading into the holds, resulting in customers shipments forgotten or wrongly dispatched. The spare aircraft will get take them to Memphis in time.

Cost
According to FedEx each flight costs $30k. Most of the time it just flies this route empty or lightly loaded. Fuel consumption is about 30% higher than for the direct path.
Cost/benefit ratio
Individual customer shipments may experience delivery delays without large consequences, or without strong reaction. This is not the case for large customers for which contracts definitely include a delivery delay commitment, associated penalties and contract termination clauses for service failures.
I guess  this $30k a day investment for this region is the quality assurance response to the evaluation of risks above, and those I'm not aware of. This is good for image, for competitiveness and for contracts renewal.

That makes sense! Especially since FedEx will want all planes at Memphis in the middle of the night to sort and deliver. So if they need the plane in Memphis, which could easily be based on how many packages will be going to Denver that night, then sending it around, bringing it down somewhere along the way if necessary (e.g., DFW) can be easily worthwhile - a little extra fuel but more flexibility and the rest of the cost (crew, amortized cost of the plane) is the same.

@757toga I would guess the commercial implications are higher for freight carriers. If a few hundred units of self-loading cargo get delayed somewhere by a few hours, you pay the minimum legal compensation and that's the end of it. On the other hand, losing a long-term freight contract with a large company that is worth millions of dollars a year, because you screwed up too often, is a different scale of problem.

@alephzero self-loading cargo - euphemism of the day. FedEx has the ultimate hub & spoke system, one which "self-loading cargo" wouldn't approve of. If FDX1311 takes a quick stop at DFW, yes it might be some high-value pharmaceuticals or electronics from big companies. It could also be a bunch of legal paperwork from small local law firms or last-minute gifts (think Mother's Day or Xmas, though those are seasonal, not every day). FedEx plans for these contingencies. The rest of the freight doesn't mind the extra 2 hours in the air, and a possible pit stop. Unlike "self-loading cargo."

Could one say, with quite a bit of a stretch, that this is a sort of "Operation Chrome Dome" for air cargo? Always having some ready-units in the air, in case they are needed.

@DohnJoe: Chrome dome aircraft where flying in closed patterns for long hours. Remote equivalent today would be air refueling tankers and airborne radars. Sweepers instead have a programmed route with a landing destination.

@mins So more a cargo air patrol (CAP) :-)

Thanks for the amazingly detailed answer! I'll admit being a bit ashamed that it seems simply googling the flight number would have led me to an answer, but you consolidated in your answer a fair amount of pieces of information from various sites. Thank you!

Is it possible to explain this "sweep flight" thing  a bit more or with a example for novice people like me? This is a interesting answer and so I thought that it Will be more easy if it's explained a bit more and there is no wikipedia article or much info. about sweep flight in Google. Thanks.

@SwiftPushkar: I'll be happy to make the answer easier to understand, but I'm not sure what kind of details will help. Can you tell me what is difficult to get or appears strange for a person not familiar with aviation?

@mins - Yes ,  aircraft unplanned maintenance and parcels left behind by the regular aircraft" part pls.

"This is not the case for large customers for which contracts definitely include a delivery delay commitment, associated penalties and contract termination clauses for service failures" - Why are those large customers so sensitive to things arriving a day late?  I can't think of anything you would ship as bulk air freight that would be ruined if it had to wait an extra day to arrive - stuff like industrial chemicals would be shipped by train or tanker truck rather than cargo aircraft, and high-value perishables like human organs would be hand-carried by fast charter jet.

@Vikki: "Why are those large customers so sensitive to things arriving a day late?" They are conducting a business or a public service, they have their own commitments, and are looking for overall fulfillment of service level agreements from their subcontractors. E.g. for the US postal service subcontracted to Fedex, penalties might be triggered when  there is more than 5% of their monthly shipments late by more than 12 hours, and the contract terminated if this occurs more than 3 times in the year. This is to illustrate.

@mins: That's pretty hypocritical of the USPS to penalise FedEx harshly for delivering things late by less than a day when they themselves routinely deliver things days to weeks late.

@Vikki. That an example, I know nothing about the contract with USPS. Individuals receiving their shipments late without compensation doesn't mean USPS large customers are not receiving damages. I know no large B2B contract without SLAs and penalties.


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Is this concept of a hybrid between a Turbine Engine and a Reaction Engine (Rocket) practical?
So, I don't understand much about engineering in general, but since this is a discussion forum, I like to think that this is just a thought exercise (because it is probably useless).
So, the idea would be to use an electric compressor that would pump pressurised air into a combustion chamber without the low pressure fan, and the combustion would happen with traditional jet fuel or other propellants (like pressurised methane?).
The idea is that the pressurised fuel would turn a small electric generator after exiting the fuel tanks, but I doubt that it would generate enough energy to drive the compressor.
Well, it would probably be terrible in subsonic or even supersonic speeds, but maybe it could be also a ramjet/scramjet hybrid (of course, with all the extra equipment it would require to do these functions).

"since this is a discussion forum"... no this is not. From What types of questions should I avoid asking?: "Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much".

With ideas like this it is always helpful to look at where the energy is coming from. Specifically the compression stage adds energy to the air and that energy needs to come from somewhere. In a conventional jet engine that comes from the turbine stage. The exact number varies but 30% of total output power seems a reasonable starting point.
So to a first order approximation, to run the four electrically compressed engines for a 747 you need enough electricity to drive 30% of 65 Megawatts or 19.5MW of electric power. Lithium batteries contain about 250 Watt hours per Kg so one hour of flight will need at least 78000 kg of batteries. For comparison Jet fuel contains around 12 Kilo Watt Hours per kg and as a bonus once you burn it your aircraft is lighter so even allowing for only 30% efficiency turning that energy into compressor power an hour of flight comes out as needing 4800kg of jet fuel*.
So there are seemingly few uses for electrically driving the compressor stage of a jet engine other than when starting it up, and even there it is apparent why most large aircraft use compressed air from the APU to start the main engines rather than electricity.
*Power number source appears to be for takeoff so cruise flight would be 30% or so lower which does not meaningfully change the outcome.


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Besides forward swept wings, which are the other most unstable wing configurations?
I know that forward swept wings are unstable and all, and that's one of the reasons they aren't vastly used in planes in general.
However, I would like to know more about other types of wing configurations that were attempted or suggested in the past that would be as unstable as forward swept wings.

What kind of instability are you referring to? Many gliders have a slight forward sweep and they're usually pretty stable.

Google "tower jumpers".  Oops, silly me.  In the post September 11 2001 world that brings up something completely different from what I had in mind.  Ok, google, "medieval tower jumpers".  Oops, silly me, for some reason that brings up inflatable "bounce houses" for children.  I give up!

They aren't necessarily unstable, except for maybe reduced yaw stability and some degree of backwards roll/yaw couple effect with extreme forward sweep (which may be good for dutch roll behaviour anyway). In the end, as long as the Neutral Point/CG relationship is good, you should be stable as far as that goes.  One of the biggest technical problems with radical forward sweep is a much higher torsional stiffness requirement than a normal swept wing, because bending increases tip incidence (wash-in from bending) instead of reducing it as with aft sweep and you really don't want that.

I have a feeling the AD-1 may fit the bill.

@quietflyer - Would BASE jumping work for your search?  Although I’m not seeing any unstable wing configurations…

How about the skydiving wing suit.  I would say that's about as unstable as you can get, but it's not really a wing as an inanimate object.

Stability is more a feature of the whole aircraft, not the wing.  But  I guess you are asking about wings that create a destabilizing pitch or yaw moment.
For yaw, the dominant factor is going to be sweep: forward sweep is destabilizing and aft sweep is stabilizing.
For pitch, any cambered airfoil creates a nose-down pitching moment which causes the Center of Pressure to travel aft as angle-of-attack is decreased, and causes the Center of Pressure to travel forward as angle-of-attack is increased.  This is destabilizing.  For aft-swept wings, washout creates a stabilizing effect in pitch, and negative washout (never actually used) would create a destabilizing effect in pitch.  For forward-swept wings, washout would create a destabilizing effect in pitch.
We shouldn't forget about roll stability.  In the roll axis, dihedral and sweep are stabilizing (tend to make the aircraft roll toward wings-level), and anhedral and forward sweep are destabilizing (tend to make the aircraft roll toward a steeper bank angle).  Anhedral is very often used to offset the dihedral-like effects created by other aspects of an aircraft's geometry (e.g. a tall vertical fin that creates a lot of surface area above the CG), but in and of itself it could be called intrinsically destabilizing.1
Since we are talking about several different kinds of stability here, and since in any given aspect, the stabilizing or destabilizing effects of the wing geometry may be offset by other aspects of the aircraft's geometry, it's kind of hard to evaluate any given wing shape to say whether it is "as unstable as forward swept wings".
If there were a specific reason to use a forward-swept wing on an airplane, the resulting destabilizing yaw and roll characteristics could be offset by other aspects of the aircraft's design, such as a large, tall vertical fin, high wing placement, dihedral, etc.  Arguably more troublesome are the structural characteristics of a forward-swept wing-- the wingtips tend to twist toward a higher angle-of-attack under heavy lift load, which further increases the lift load and twisting moment, and the bending moment on the wing spar(s).  A forward-swept wing must be engineered to have a great deal of torsional stiffness to avoid problems in this area.
So in summary:

I know that forward swept wings are unstable and all, and that's one
of the reasons they aren't vastly used in planes in general.

It may be "one of the reasons", but it's actually a rather minor one.  The main issue is structural-- the torsion issue.

However, I would like to know more about other types of wing
configurations that were attempted or suggested in the past that would
be as unstable as forward swept wings

A wing with significant anhedral and little or no sweep would seem to fit your description, at least if roll stability is a parameter of interest, and we are looking at the wing alone, in isolation from the rest of the aircraft.  One example of such a configuration would be the wing used on the Lockheed F-104 Starfighter.  Another good candidate would be the wing on the McDonnell Douglas AV-8 Harrier II-- it has some sweep, but appears to have slightly more anhedral than the wing on the F-104, and certainly has more span, giving the anhedral geometry more leverage with  which to generate a destabilizing roll torque.
Footnotes:

In the interest of brevity, we'll avoid opening the whole "can of worms" relating to complex forms of dynamic instability that can actually be exacerbated by too much roll or yaw (directional) stability, such as "Dutch Roll" oscillations (exacerbated by too much "dihedral effect", whether from sweep, actual dihedral, high wing placement, or other aspects of an aircraft's geometry), and spiral instability (exacerbated by too much yaw (directional) stability.)

Adding slight forward sweep to a slow flying glider minimizes "torsional" issues (and potential flutter), and, when fuselages were "boxy" with more surface area, provided a useful tool to reduce excessive yaw and roll stability.

@RobertDiGiovanni -- re "provided a useful tool to reduce excessive yaw and roll stability"-- I dunno, I sense a new ASE question in the making here-- as far as yaw stability goes, it's very easy to shrink the fixed portion of the vertical fin down to virtually nothing-- the Scheibe Bergefalke went pretty far in this direction, and while still perfectly flyable, I wouldn't describe the resulting flight characteristics as ideal.  Nose wanders off to one side (sideslip), and if you attempt to hold heading with ailerons only (no rudder input), there appears to be virtually zero tendency (ctd)

(ctd) Nose wanders off to one side (sideslip), and if you attempt to hold heading with ailerons only (no rudder input), there appears to be virtually zero tendency for the nose to swing back into alignment with the actual flight path / airflow, the glider just keeps flying along happily with the yaw string streaming crazy sideways --  --  thanks in large part to adverse yaw from deflected ailerons-- (wings aren't level and aren't seeming to want to come back to level, due to roll torque created by interaction between sideways airflow and dihedral wing geometry (plus high wing placement))--

Even the much-loved Ka-6 has some tendencies along these lines

Well, can accredit that to the large "schnozz", and less dihedral.  Would agree that forward sweep would not be necessary with other options available.

Ziv Nedivi and Yehoar Gal flew an F-15 with only the left wing still attached.  It was not as unstable as you might think.  A maximally asymmetric wing will have the most torque on the fuselage.  That will cause a pronounced spinning tendency.
I suppose you could make the situation worse by removing one wing from an aircraft with a lower wing loading and higher aspect ratio.  Maybe one of these gliders with a wing removed?


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is there a way to convert speed into g-force, or do you need acceleration too?
kind of a stupid question, just wondering.

Calculation of the G-force on Physics.SE

This is a physics question, but.
Speed has no relation to g-force. One g is the vertical acceleration one feels when standing still on the Earth's surface, and is equal to –9.8 meters per second per second (an acceleration, you will notice). When one changes one's velocity, i.e. accelerates, one experiences a g-force acting in the opposite direction of the change in velocity: If you go faster forward, you feel a g-force pushing you backward, and vice versa. If you go faster upward, you feel a g-force pushing you downward, and vice versa.
So if one's velocity is constant there is no g-force felt other than the standard one g due to Earth's gravity. Note that I have used "velocity" throughout—velocity specifies both an object's speed and direction. It is possible to maintain the same speed while changing velocity, for example in an uncoordinated turn; it is also possible to keep going the same direction and change velocity by changing speed.
Now it is true that the same control inputs, when going at different speeds, will result in different changes in velocity, ergo different accelerations, ergo different experienced g-forces. Compare a fighter pilot pulling the stick back 10% while flying at 150KT with a fighter pilot pulling the stick back 10% while traveling at 500KT—quite different!


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Gain Extra force from Weight in Slopped Runway
No doubt that skiing sports is great especialy when your weight is the main force which speed up your motion in snow.
I was asking myself, why we don't use the same concept for huge planes & reduce some of fuel consumption during takeoff stage on runway.
Typical Boeing 747 jet burns about 5000 Gallons in takeoff until reach cruise speed. It's about 10% of their fuel capaccity.
Is it possible in constructional side to build slopped runway like what we see in right corner photo OR it has some risks ??
Is it worth it or there are tradeoff with other considerations?

Most of that 5000 gallons must be climbing to cruise and unaffected by the runway.  Closer to 850 gallons for takeoff roll and climb to 3000 feet.

What would you recommend to stop an aircraft on a descending runway? Snow plow or parallel?

The main reason is a bit different from what is mentioned in the “duplicate” question: it would not reduce the energy required, so it would not reduce the fuel consumption. By rolling downhill, the aircraft exchanges potential energy for kinetic energy. But then it has to climb back up, and further to the flight level, which means it has to gain the potential energy again, and it's the engines that have to provide it. So in the end the engines have to provide the same energy and there are no savings to be had.

… it does allow shorter runway, which helps in places like Courchevel or Lukla where there isn't room for create a longer nor flatter runway. But it comes with its own challenges, making flat runway preferable where possible.

@mins land uphill.  Except of course we can't usually assume still air so we can't do that.  Airfield that have one-way runways or similar constraints have a lot of unplanned downtime as at Tenzing-Hilary

@ChrisH: I believe the context is takeoff only, I was referring to aborted takeoffs, e.g. because there is an engine failure before V1.

@mins Ah OK.  I was thinking about the planes having to arrive before they could depart. Certainly Courchevel looks like you might be getting in a bit more skiing than you'd planned if an engine fails on departure

Believe it or not, the ramp would lead to a shorter take-off but would burn more fuel.  A climbing aircraft (to  regain altitude) must also burn fuel against lift drag as well as angle of climb.  A rolling aircraft only burns fuel against the grade with minimal axle friction.  If the runway was 3 miles long and 300 feet up hill, a light aircraft could save fuel by taxiing up the first 2 miles, then taking off uphill!


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Extending the wing of commercial plane
Question No.3
I was asking my self question related to the payload capacity which is:
*

If we want to carry more passengers & cargo containers, so it's
obvious we should increase the wing area

*, but as we know the size of airport road is limited to span of wing approximately 80m, so why we dont extended it longitudinally ??
Is there limitation also due to :

Availability of many safe doors as much as possible.
Manufacturers dont like to change the old fashion of airplane design, (A380 was made an exceptional development in avaiation history by making 2 decks).
or it has relationship with Reynolds number as in wind tunnel test if wing chord is 1m @ density 1.2 Kg/m3, then in high altitude, the chord can be 4m @ density 0.3 Kg/m3 as we want to have the same Reynolds number.
Structural or Economical issues.

So, what's the right answer ??

Your username is certainly appropriate to the question...

Still waiting for your airfoil that puts supercritical ones to shame.

Induced drag is the drag created by anything that's creating lift, such as a wing. It's caused by the high pressure air underneath the wing flowing around the end of the wing into the low pressure zone above it. This creates a vortex that in turn creates drag on the wing.
The formula for the coefficient of drag is
$$C_{di} = \frac{C_l^2}{\pi A e}$$
where $C_l$ is the coefficient of lift, $A$ is the aspect ratio of the wing (i.e. the length divided by the chord), and $e$ is the efficiency of the wing's lift (calculated based on its lift distribution; for a perfectly elliptical lift distribution, $e = 1$). Since $A$ is in the denominator of the equation, the bigger it is, the lower the amount of induced drag.
TLDR: Long, thin wings generate less drag than short fat ones.

you are right in aerodynamic side but someone will comment an inverse question: why we dont decrease the area of wing, so we can decrease the induced drag for recently plane. Definitelly thats mean lowering the capacity of passenger. I think that it has not direct relationship with induced drag because we know that it will be only high in low speed (take-off). Also, increasing area will let us have more space to store fuel which increase therange of flight as double. What i thinks it has relation with reynolds number which limit our chord or the price of ticket -economical

@peter_kampf If you have account either in Linkedin or Facebook, you may follow hashtag of my development there: #Miracle_Airfoil_Project     - All questions that I have made here is related directly with my new design & until now we reach takeoff phase performance which let us have CL_max=2.7@AoA=18deg for V=60m/s and still we waiting for CFD test of other lower takeoff speeds. I don't thinks it's appropriate to post about that matter here as it's question forum only, but I will not deny, it's helpful forum let me to find experts like YOU & other which oneday I will thanks this great website

@MiracleAirfoil You misunderstand. It's not the area of the wing that's important for that equation, but specifically the aspect ratio. If you halve the chord but double the length (all else being equal), the area (and therefore lifting ability) stays the same, but the aspect ratio goes up by a factor of 4, reducing drag by that same factor.

This answer has it spot on. A long-chord, low-aspect-ratio wing is aerodynamically inefficient, and that wastes expensive fuel. For the 777X, Boeing therefore calculated that adding folding wingtips was a more cost-efficient way to fly more payload from a standard terminal: https://en.wikipedia.org/wiki/Boeing_777X

@HiddenWindshield 1) Its my mistake that I dont share with you my assumption to agree for whats next. I have assumed that span of wing & max lift coefficient are constant (max span in airport = 80m), so area (a) is inverse relationship with aspect ratio (AR).
So, induced drag formula, increasing area will lead to increase the induced drag by the same amount.
But, we forget that airplane manufacture doesn't care too much to obtain high speed, they want attract customers (airliner agency) which is related to the price of ticket or you may say [Liter/(Pax.Km)]

@HiddenWindshield 2) So,when we increase the area by double, we increase the drag eventually, but the number of
passengers will be also doubled, then rather than carry 500 pax in one plane, we can carry 1000 pax with the same price of ticket.
Concorde has been failed - even it has high cruise speed - obtain percentage of market potential of commercial transportation because the price per passenger is too much high for business men & women. Also, to pay 10 times the ticket price of ordinary plane only to reach your destination in less than half hour is not really worth it.

@HiddenWindshield 3) As i understand from Concorde, it's not the fuel issue which let to have high price, but the special & expensive material which should withstand high temperature. I dont know how company like BOOM will solve this problem now. Maybe they make market survey & found there are certain people would like to pay more money but not like Concorde palne if it's possible

@MiracleAirfoil A faster airplane can make more trips per day, carrying more passengers. So, yes, the airline industry does care about making planes fast. That is, after all, why supersonic passenger aircraft were even attempted in the first place. Also, if you double the area, you DON'T double the number of passengers. You're forgetting to take into account the added weight of the wing and fuselage structure. So, your $L/P_xK_m$ equation is going to go up, not down, with this wing.

@HiddenWindshield Assume that we have 2 planes each have total gross weight of 250 ton with L/D =20. This 2 planes if it is emerged into ONE plane we will have new parameter which is: Gross weight = 500 ton & L/D=10 (in worst scenario). So why Lift-drag ratio is decreased ? The reason for that our span is limited in the airport up to 80m, so aspect ratio will decrease as a result of increasing Area. In point view of fuel consumption, it's BAD choice, but the advantage of doubling the number of passengers or cargo containers may equalize it as the same of original plane for 250 ton with L/D=20

@MiracleAirfoil You yourself pointed out the desirability of minimizing the $L/PxKm$ equation.

@HiddenWindshield You are right expert. L/D should be constant for any increase in number of passenger so they will pay the same value of L/Px.Km. Thanks alot for giving me your time. :)


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Why 737 NG Flight Director need to be Recycled?
When FD switch is on, FD is on.
When FD switch is off, FD is off.
But when FD switches are recycled, FD switch on, but FD bars are not shown during approach. Is this due to approach FD logic? If so, what is the approach logic and when does it arm?
I understand everything about availability of FD bars during TOGA and Alt Acq.

At the moment when both switches (and the autopilot) are off, there is no active guidance mode (like HDG, V/S, LNAV, LVL CHG, APCH, etc). (In fact that's typically the reason to turn them both off, to disengage the active mode/s.)
Then after they're back on, they only display when both a lateral (heading, VOR, LNAV) and a vertical (level change, vertical speed, altitude hold, VNAV) mode are selected. Until one each has been selected, a single-cue FD remains out of view; a dual cue FD will show a cue for whatever mode is active.
For completeness, both Approach and TOGA are lateral and vertical modes.
If you're on an approach & switch both FD switches off then back on, you'll only get the bars back after you also press APCH to arm the approach mode. The LOC will capture, and then the GS, and at that point you'll have the FD bars again. But you get those only after you arm the mode.
One of the reasons we might cycle the FD's is if a TR unit has failed, which causes problems when the bus separation relay opens at GS capture.  We get the relay closed again by "removing" GS capture by clearing the GS FD mode by cycling the FD switches. Now your electrical system looks normal, but if you re-engage APCH mode, you'd get the same problem back again. So, don't do that! Either fly the approach visually, or in LNAV/VNAV, or go around & get a new plan. But the idea is to NOT have APCH mode active, in this case. (Or, at most, you'll have one pilot with an FD in APCH mode, but the other pilot won't have an FD, due to the bus separation.)

Why no FD bars when you recycle them?

No active mode = no bars. Both FD's + AP off means all modes are cleared, and turning an FD on doesn't in itself activate any modes.


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What limits automated direction finding from working over oceans?
Automated direction finders are only reliable in relative close proximity to NDBs, due to various adverse effects. In particular, the night effect limits their useful range for navigation independently of the NDB transmission power.
However, there was at least one historical navigation system, Omega, that has offered global coverage through high-powered VLF beacons.
In contrast to NDBs and ADFs, Omega was based on hyperbolic navigation, and in particular compared the phase difference between signals received from different beacon stations. These stations were transmitting in the VLF band, with their transmissions reaching over ten thousand kilometers due to the unique propagation characteristics of that band.
What I dont understand: If there is a capability to transmit a radio signal that far, why can't the same principle be used to determine the direction to the transmitter using an ADF, using radio direction finding principles, and then determining one's position by intersecting the determined bearings to the various transmitting stations as is possible using NDBs?
Is there something unique to the VLF band that makes it less prone to the distortions affecting NDBs, or is it subject to them as well, but the phase comparison mechanism is somehow less prone to errors and measurement ambiguities than radio direction finding would be over the distances involved?
Has Omega just solved for these problems by modeling VLF propagations sufficiently?
Or is it something else entirely?
Note: This question was originally part of Was radio navigation used for oceanic crossings before GPS? If not, why not?, but I've spun it off of that for clarity.

ADF radio sets tuned to NDB transmitters use frequencies just below the AM radio band, which do not constitute VLF signals. Their useful propagation distance is therefore short, typically a hundred miles or so, for a transmitter radiating a couple hundred watts. 500 mile range would require 50,000 watts or so and a huge antenna array. Reliable 1000 mile range needs 100,000 watts or more and an enormous antenna farm at those frequencies.

What I'm wondering is: Could they be extended to use VLF beacons (existing ones used for other purposes, like Omega, or new and dedicated ones, for NDB-like usage)?
Or would the precision of such hypothetical "VLF ADFs" be equally bad or worse at oceanic distances, making a more complicated system (based on hyperbolic navigation or ranging, potentially via complicated propagation correction tables such as those used for Omega) the only option?

sorry, no idea. -NN

@lxgr with the introduction of INS in the 1930s (or early 1940s) the need for the very limited scenario of oceanic traffic for the long range NDB disappeared, so the systems were abandoned in no small part to save cost. Similarly, most short range NDBs have been removed to make place for VORs, and now many of those are being removed because GPS is commonly available (and no, I'm not going into the wisdom of those decisions, apart from the obvious financial savings not having to maintain those beacons)

@jwenting That makes sense to me! To make sure I get the point – the idea is that INSes became accurate enough quickly enough to make radio beacons uneconomical (and maybe even less accurate given typical travel times and NDB angular error rates, I suppose)? If so, I think that would make for a good standalone answer!


text:
While waiting for a delayed outbound flight how can I guess where's the inbound plane?
This summer in Mykonos our flight back to FCO was delayed and nobody at the airport could communicate reliable estimates on when it would be.
In that case I managed to find the FCO-JMK inbound flight on FR24 and saw it left FCO with 2 hours delay, so our own would have been at least that.
In bigger airports with more connections you might not be able to guess which airplane will become yours.
I guess the registration number of the plane assigned to my flight will be available some time (maybe a few hours?) before as per the flight plan.
Is there a reliable public accessible source for that?
When I have the assigned RegNum I can use plane finder, FR24 or whatever to see if it's on its way and have a rough estimate of when it will land and then be available for my flight.
Apologies for my English.
Thank you

This should possibly be better on Travel.SE

If there is just one line by that airline, you can guess it easily. But if they have many flights there, the dispatcher can reassign the planes at any time as the dispatcher, and only the dispatcher, has the information needed to do the scheduling to minimise the disruption to the overall schedule. So until the dispatcher says something, even the other airline staff does not know.

Thank you this makes a lot of sense and it might be a good reason for which sometimes nobody seems to know. But in my Mykonos example once I got the inbound flight number and THAT was delayed by 2.5 hours I had a good estimate :)

My understanding is that this depends on the airline, with some publishing which aircraft will be operating which flight in advance (though I don’t quite know through what means), while for others you will get the info only once the flight is ready to depart. IIRC FlightAware (or is it Flightradar24?) has a « track incoming flight » button for this purpose, not quite sure how reliable that is. Maybe it’s just heuristics...

It's difficult to find out what the inbound aircraft is, unless you are at a small airport and/or the airline you are flying on is only operating a few routes from the airport. When you are at a big airport and/or the airline is operating many routes there, you have no way of telling where your aircraft is. For smaller airports, you can effectively find the inbound aircraft by using the appropriate filters in FR24.
Note that in some cases the aircraft planned for a flight can be changed last minute. This typically happens when aircraft are significantly delayed inbound to their hub / main base and the airline decides to minimise the outbound delay costs by assiging a different aircraft.

Some tools provide this information. For example, I have found FlightAware to be generally accurate. Some airline's services (online or mobile) services also provide the information.
Information might well not be available to the public for some airlines.

Is there a reliable public accessible source for that?

No, there isn't. The registration is listed in the ATC flightplan, which is not publicly available.
Some airports do publish the registration of flights, enabling you to find the right aircraft using a bit of investigative skills. One such airport is Frankfurt, and I if I recall correctly, at least some other German airports do the same. If you download the mobile app for the particular airport, you will be able to find the registration. FlightRadar24 also sometimes have it available, and sometimes the airline might even show it somewhere (app, website, ...)
But a reliable public accessible source? No, unfortunately not.


text:
What causes fuel gauges to be untrustworthy?
I went on my first flight yesterday (Cessna 152), and I was told not to trust the fuel gauges and to check fuel levels manually. The instructor said that the only fuel gauge reading that I can trust is a 0.
Why is that exactly, and what can be done (or what has been done/studied) to improve their accuracy? Is there some fundamental technical limitation that makes better accuracy difficult to achieve?
Do automobiles have more or less the same issue, or is the inaccuracy more prevalent in aircrafts?

The following may not be 100% accurate for aircraft. Usually, fuel gauges use a buoyancy to determine the level of the fuel. With changes in attitude, there may be inaccuracies due to fuel sloshing to one side or another. Also, because there are moving parts, they can fail more easily than, say, a pitot tube.

The problem is small airplanes use automotive style rheostat (variable resistor) fuel senders, with the float in the tank, and these just aren't reliable enough to be trusted with something potentially life or death like fuel quantity, and they aren't particularly accurate in the first place.
Here's the thing... In a car you just drive around going past gas stations constantly until the gauge is low.  Imagine you were operating a car in some remote place where the gas stations were 300 miles apart and if you run out of gas short of the next gas station, the road is full of hungry alligators equipped with Slim Jims, making your door locks useless, and you, dinner.
Well, your tank is good for 600 miles and it's half full on the gauge.  Maybe good for 300 miles, maybe not. Are you willing to trust the automotive gauge now?  This is what faces you as pilot any time the tanks are not full, in an airplane with 40 year old fuel senders, that maybe sits around for extended periods.
Bigger airplanes use fuel quantity measurement systems that use "Capacitance Probes", hollow tubes with a center rod, and a current is passed through and the capacitance measured by the fuel level in the tube that completes the circuit between the outer tube and the inner rod.  A computer will monitor the levels from several probes arranged around the tank and they provide a very precise and reliable fuel reading.
There are capacitance probe senders available for homebuilt aircraft, but I'm not aware of any certified units that can be installed in something like a 152.  If available, that would be a good solution, although I'd probably still not be willing to trust it 100%.
Best thing is to use a dipstick and be aware of your fuel consumption/time. During training and renting airplanes, people get into the habit of always going somewhere with full tanks, but later on, especially if you are flying commercially, you are going to have to fly a lot with intermediate quantities, excess fuel being unwanted ballast, so you need to get into the habit of knowing and tracking fuel quantity.
Impress your instructor by showing up with something like this and you will always know exactly how much fuel you have.  If you're cheap and refuel the plane yourself you can make your own with a wood stick if you have a way to get all the useable fuel out of the tank, then add 5 gal at a time and marking your stick appropriately at each level.

Re last paragraph; just don't drop it into the tank by accident.  Yes it is possible if it starts sliding sideways out of your hand.   Been there, done that.   Came back with two dowel rods and used them like chopsticks to fish it out but it sure took a good long while.

I used to use a wooden ruler and finally glued a cross piece to it to prevent that after I nearly dropped it in lol.  Nowadays I use a stick that is a tube in a piece of alum channel and I hold my finger over the top of the tube to get the level, and it's simply too long to fit in the tank that way.

I'm guessing that senders is a typo for sensors in this answer?

Newer planes use magneto-resistive sensors.  The float arm uses a magnet in the fuel tank, an externally mounted sensor reads the magnet position to sense the fuel level. No electricity running thru the tank, and the signal can be calibrated for more accurate readings.  These are retrofittable into older planes as well (I did); the cost to repair original resistor float guages are as much as new modern equipment, so it makes sense to change over and get way from 45 year old corroded floats. https://ciescorp.net/documentation/comparative-fuel-sensor-technology/

@TannerSwett no actually fuel sender is a term that used to be used back in olden times.  And... when I looked at CrossRoads' link, it says "fuel sender", so not so olden I guess lol.

Huh, interesting.

The other two answers are very good but leave out a key point, airplane fuel tanks are not always level and since liquid will slosh around and level its self (un an unaccelerated situation) the gauge can only be accurate under certain conditions. In a car you are always more or less on level ground often going a constant speed. In an aircraft for a given speed you pitch will change based on load configuration. You also need to bank to run and pitch up/down to climb and decent. Often the sender is put somewhat in the middle of the tank to mitigate the issue a bit but that is only so effective.

(source)
Turbulence, small course corrections, and pitch/power changes will all slosh fuel around and ultimately make the gauges inaccurate.

Sooo few airplanes have that kind of system anymore. Certainly not anything with an electrical system.

@CrossRoads it was merely the best picture i could find to show how floats work. Most GA planes still have a similar float that operates a rheostat.

A sendor takes the sensor reading and sends it to a device like a JPI EDM900 which is calibrated with the sending unit.  Upon filling the tanks, the pilot can tell the EDM900 how much fuel is in each tank to start a flight. The EDM900 than displays the fuel level as the flight progresses, can do the math to display fuel used, fuel remaining, show fuel flow from an appropriate sensor (in my case, feeding into the carburator), can send fuel information to a GPS to display a flying range available, can warn of an empty tank, and can tell you how much to an empty tank.  Much more info than just an old float gage that was only accurate when it was empty.
The info I see is similar to this, with the addition of carburator throat temperature, and just 4 cylinders vs 6.
I would say the fuel management/totalizer functions are similar to what modern cars have, with real info provided vs just an idiot light for many of the functions.

Do you trust it enough to no longer dip the tanks if going somewhere with close to minimum fuel requirements?

I'm not sure how this answers the question of "what causes fuel gauges to be untrustworthy". Am I missing the obvious?

Nope.  I still dip the tanks before a long flight just to be sure, or at least look to be sure the level is where I last filled it to.  I did a 3 hour flight Saturday with ~4.4 hours in the tanks. Upon getting close, I was seeing warnings about low fuel. Dipping the tanks before re-filling, I had 3 gallons on one tank and 9 on the other, so an hour+ still available. I still have some learning to go on resetting it upon filling the tanks or filling to a known quantity.  Seeing the fuel flow and the cylinder temps is great for leaning the engine out, better than a single stock exhaust temp gage.

It answers the 2nd question of What can be done to improve their accuracy.

In my bush flying days in a C-180 on floats the trips were around 1 to 1.5 hours round trip + 30 min reserve, and payload"s everything,so I rarely left base with more than 2 hours total, maybe 20 imperial gal, usually between 1/4 and 1/2 on the gauges.  On the way back getting close to base, if I'd had to deke around weather, the gauges would be showing the sender floats bouncing on the tank bottoms with  ~5 imp gal per side remaining and I'd be saying to myself "trust your dipstick". I'd land and dip the tanks at the dock just to see and sure enough I still had 30 min to an hour's fuel.


text:
What kind of gauge is this?
I acquired a small number of vintage (WWII era) aviation gauges. The others I have been able to successfully identify, but this one is a bit of an odd duck. It is marked Kollsman Type C-14, Part No. 586BK-101, Serial No. AC-42-781. I believe this places it as manufactured in 1942 and is a Kollsman Altimeter. The face of the gauge, however, is clearly marked as Gage Vacuum Pressure In. of Mercury and embossed with AF US ARMY. Could this just be an altimeter that was converted to a barometer for some reason?

This is likely a suction gauge. Specifically, this gauge must read between certain values to assure the pilot that flight instruments operated by an engine-driven suction pump are operating correctly.  For instance, for the P-51D, the flight training manual states the following -

The suction gage shows whether the vacuum pump is providing proper vacuum for the system.  If this gage reads more than 4.25 or less than 3.75, you know that the vacuum instruments are not functioning properly and are not giving reliable readings. Normal suction reading is 4.00.

The manual states that these instruments, namely, the flight indicator, the bank-and-turn indicator, the directional gyro, and the suction gauge, are all operated by the engine-driven suction pump.  That this instrument is embossed AF US ARMY indicates that it was a part of the flight instrument group for a military aircraft possibly used during the Second World War.  (Note: The government spelling of gauge is taken as gage.)

Looking at the gauge in the OP, I do not see any needle position where the range 3.75 - 4.25 would be clear. I believe that these instructions do not pertain to the gauge in the photograph.

The gauge's first rotation is 0 to -4 inhg, and the second rotation is -4 to -10. -3.75 to -4.25 would be roughly straight up, assuming suction was >0. It is slightly unnerving that "broken" is indistinguishable at a glance from "normal running condition".

This is a Vacuum Pressure gauge that would have been used to indicate the system vacuum pressure in an airplane.


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Flying a twin engine aircraft with 2-axis control
I believe a twin engine aircraft, with 2-axis control (i.e. Rudder and Elevator), under normal circumstances is flyable.
But is it safe at all?
If one of the engine dies, will the rudder alone be sufficient to counter the consequent  roll/yaw?

Define safe. This would be a non-starter for Part 23 and 25.

@Jimmy I mean "save" literally. What other definition do you need?

I don't think that you can have rudder/elevator only control for a twin in an engine-out scenario. The problem just isn't flying straight, you need to be able to get somewhere too. I don't think you have enough authority to both counter roll/yaw and be able to navigate, especially IFR (spatial disorientation in an extended slip, you need more authority to get out of a bad situation). The only situation where this may work is with in-line twins or twin-jets that have both engines mounted close together.

@dammy999 If your aircraft does not meet the minimum controllability requirement mandated by Part 23 (14CFR for example), do you feel safe? If so, what is your min threshold?

It sounds like you're thinking through a hypothetical exercise for amusement (or for a bet with another pilot) where you lost aileron control in a twin and were trying to maneuver on rudder alone, then one of the engines quit.  Is that about it?

@Jimmy I haven't read the "Part 23" stuffs, so I don't know anything about the safety measures that were discussed in it. I've been searching for the PDF version of the document, though.

@JohnK It's not really about competition with another pilot. I saw the "Bloop 4" ultralight aircraft (www.m-sandlin.info), so I thought using two engines (instead of the one used) will still work.

You should incorporate that sort of specifics in your question.  Most of us here are thinking through Piper Seminole-like scenarios which is far away from the information you are after.

@JohnK Oh, I'm sorry for that. I'm a newbie on this site. Do accept my apology.

No worries! We aim to please.

Isn't this a certification requirement for non-centerline-thrust multiengine aircraft?

For an ultralight with rudder only roll control, which achieves lateral control with a lot of dihedral to create a really strong roll/yaw couple, the issue would be how much reserve rudder authority is there once enough has been applied to counteract asymmetric thrust.  That would determine how much roll rate you could still achieve trying to yaw toward the dead engine. If the engines are reasonably close together, it would probably be flyable but you would have a lot less turning authority into the dead engine than away from it.  In the end, you would have to go try it to see what happens, or get someone to do it with an RC model version.
I think your problem is more to do with simply single engine performance.  If one quits you are now at 50% power.  Using two engines doubles the chance of engine failure, and if you can't climb decently on a single engine, you're still in a glider after a failure, just one with a flatter glide.  IIRC, the Lazair twin was just able to maintain level flight on one engine at lower altitudes, and it was pretty clean as ultralights go.  The Bloop being a lot draggier, you would probably need more total HP that it uses as a single, to have decent reserve power when one fails.